ABSTRACT: To gain insight into the atomistic details of membrane fusion induced by fusogenic peptides, molecular dynamic simulations of synthetic peptides, derived from viral fusion proteins, contained in lipid bilayers were performed. A 20 amino acid peptide from the N-terminus of the influenza HA fusion peptide (WT20) assumed the oblique orientation at the interface between water and the membrane made up of dipalmitoylphosphatidylcholine (DPPC)/palmitic acid (PA), as reported previously for different membranes. Simulations of WT20 embedded in bilayer membranes made up of dioleoylphos-phatidylethanolamine (DOPE) and DPPC/PA showed a positive curvature-inducing effect, whereas WT20 showed a negative curvature-inducing effect on a DPPC bilayer. In phase re-constitution analyses starting from a random mixture of DPPC, PA and water molecules, WT20 weakly stabilized an inverted hexagonal phase. In the latter analyses WT20 preferentially assumed a transmembrane orientation as opposed to the interfacial orientation, regardless of the phase to which the system settled (lamellar vs. inverted hexagonal). In another set of analyses using systems containing a water layer between the apposed DPPC/PA (and DOPE) monolayers, the behavior of WT20 during the formation of an intermembrane connection (or stalk) was examined. Comparison among the mutants supports a view that the oblique orientation of WT20 facilitates the perturbation of the lipid-water interface and the stalk formation. Taken together, these results imply that the influenza HA fusion peptide can have substantial effects on the membrane curvature and can assume a wide range of orientation/position in membranes depending on the local environment of the lipid/water system. Its movability and oblique orientation appear to be associated with its ability to perturb membrane/water interfaces.